Wave signal frequency-changing system with constant current biased frequency changing device



o-oAMPL|FlER VOLTAGE DETECTOR -CHANGING SYSTEM WITH CONSTAN F. BELL ETAL Filed Dec CURRENT BIASED FREQUENCY CHANGING DEVICE LOCAL osc.

'F/gZ WAVE SIGNAL FREQUENCY June 2, 1953 ll'WEfi/TORS JOHN F BELL ROGER M. NORDBY THE/f? ATTORNEY W Mm 2. ||ll IIIIJ Patented June 2, 1953 WAVE SIGNAL FREQUENCY-CHANGING SYSTEM WITH CONSTANT CURRENT BIASED FREQUENCY CHANGING DE- VICE John F. Bell, Glenview, and Roger M. Nordby,

Skokie, 111., assignors to Zenith Radio Corporation, a corporation of Illinois Application December 12, 1950, Serial No. 200,457

Claims. (01. 250-20) This invention relates to an improved wavesignal frequency-changing system and especially to one which may be employed as a modulator or mixer in a superheterodyne type of receiver to derive an intermodulation product of a pair of signals. The term wave-signal frequencychanging system is a generic expression which is used herein to include detectors for demodulating an amplitude-modulated signal as well as frequency-converters or modulators because both modulators and detectors are devices for altering or transposing the frequency of an incoming signal.

One type of modulator previously used as a mixer in a superheterodyne receiver includes a tuned input circuit coupled to a conductive circuit element having a non-linear voltage-current characteristic, such as a germanium crystal diode. A local oscillator is also coupled to the diode and an intermediate-frequency signal is selected therefrom by a tuned output circuit. In order to achieve high conversion efficiency, there is usually applied to the diode a static voltage bias of such magnitude that local oscillator signal excursions traverse the portion of the voltagecurrent characteristic of greatest curvature. Although generally satisfactory, this arrangement is subject to certain deficiencies which result in variations of modulator performance as will be apparent from the following.

If the voltage bias is fixed at some optimum value for a particular diode element, that value may not be correct for another element. This is understandabl since the characteristic curve under consideration is an exponential function and the section of maximum curvature usually occurs at different voltages from one diode to another. Consequently, unless provision is made for adjusting the static bias, operation at the highest efiiciency may not be attained and increased input signal excitation may be required if a given output signal intensity is to be maintained.

Moreover, a shift in the operating region effectively results in a shift of average diode impedance. As a result, the loading effect on the input and output circuits of the modulator is not the same for every diode element or every level of local excitation and the overall band width of the modulator may be adversely altered when a germanium diode is replaced or when local excitation is changed.

It is an object of this invention therefore to provide an improved wave-signal frequencychanging system that avoids one or more 'of the deficiencies of the above-described arrangement.

A further object of the invention is to provide an improved wave-signal frequency-changing system the performance of which remains essentially constant over varied operating conditions.

A Wave-signal frequency-changing system, in accordance with the present invention, comprises a two-element predominantly unidirectionally conductive crystal device having a non-linear voltage-current characteristic. The system further includes a source of constant current connected in series with the two-element device for establishing a static current bias therein in order to fix the operating point of the device at a particular part of its afore-mentioned characteristic. The constant current source comprises a source of potential and a series impedance having a, value very much greater than the operating impedance exhibited by the crystal device. Means are provided for applying to the two-terminal device a wave signal to be changed in the frequency spectrum and means coupled to the device are provided for deriving therefrom an output signal corresponding to the wave signal, but changed in frequency.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a circuit diagram, partly schematic, of a complete superheterodyne receiver including a wave-signal frequency-changing system embodying the present invention;

Fig. 2 is a graph representing certain operational characteristics of the circuit of Fig. 1; and,

Fig. 3 is a circuit diagram of another embodiment of the invention.

Referring now to Fig. 1, the superheterodyne receiver there shown comprises, in cascade, an antenna circuit II], II, a modulator [2 constructed in accordance with the invention as will be pointed out more fully hereinafter, an intermediate-frequency amplifier l3 which may include one or more stages of amplification, a detector M, an audio-frequency amplifier l5 of one or more stages, and a sound reproducer 16. A local oscillator I1 is coupled with modulator l2 and derives its excitation potential from a source represented by a battery l8, the negative terminal of which is grounded. This battery is also connected with stages l3 and IE to supply space current to the electron-discharge devices included in those stages, as exemplified in the drawing by stage [3. One or more stages of radio-frequency amplification may be interposed between antenna H), 'II and modulator l2 and automatic amplification control may be obtained in well-known manner by a suitable connection from an AVC potential source, which may be included in unit 14, to one or more of the stages of amplifier It. All of the components of the superheterodyne receiver, with the exception of modulator 12, may be of conventional construction and th general operation of such receiver is well understood in the art so that a detaillid description thereof is unnecessary. In :brief, however, a signal is intercepted by antenna Gi cuit I0, ll and applied to modulator l2 wherein it is heterodyned with the signal from local oscillator I! and converted into an intermediatefrequenoy signal. This latter signal is amplified in intermediatedrequency amplifier i3 and is applied to detector [4 wherein the audio-frequency signal components are derived. ilhe audio-frequency signal components, in turn, are applied to audio-frequency amplifier 15 wherein they are ampiified and delivered ,to sound re- .producer to for reproduction.

Referring now more particularly to the circuit embodying the present invention, waveesignal frequency-changing system l2 includes an input transformer having a :primary winding i9 coupled withaantenna 1,! and a secondary winding .29 connected in parallel with a tuning'condenser 2.! CoilZil andcondenser .2! cons 'tute a-resonant circuit for selecting signals of a given frequency .as is -.well understood in the art. One end of .coil 2011s groundedand a tap lfiflnear-the ground- ;ed end .is connected to the (cathodeoi a vtwo-ele- .-ment, predominantly unidirectionally conductive .frequencyaohangingdevice 22 having :a non- :linear voltage-current characteristic. For pur- ,posesof the presentdiseussion, this device may ibe;considered@as-a gerinanium crystal diode. Al-

though a germaniumdiodeelement is.conduc tive .in both directions, vits conductivity .in one direction is-verymuch greater-thanin the other and .in practical applications, the device isoperated to .take advantage of the greaterconductivity ,in .one direction. Hence, vdiode ,22 is -;considered predominantly.unidirectionally-conductive.

The anode of ,diode, 22 ds connected .to. one ter- -minal of a pi,ckup coil 23 inductively coupled .to a coil 24 included in the frequency-determining circuit-of oscillator .51. lThezother terminal of pickup coil 2.3;isconnected-to one terminaloi an intermediateefrequency coil 25 the other terminal of which-is'connected .to-thepositive terminal of battery J3 through a substantially resistive impedance 2,6 having a high resistance relative to theimpedanceof diode22. .Thejunction of coil 25 and resistor {25 is bypassed to ,groundaby a-cond-enser 2l whiclnhas-a low impedance for the ,signal ,freguencies ,supplied .to and developedinstage 12. A condenser 2.8 in shunt. to coil 2.51:xripletesZ a resonant circuitwith .coil .25 .tunedito the intermediate frequency of .the receiver. ,--A ground connection and a connect-ion through apot pling condenser 2.9..connect stage l? with stage! It.

The resonant linput circuit comprising ,coil :20 {and condensen2 Lconstitutes. means; for. -apply- .ing -,to ;device 322 an ,input ;signal -,w,hich is to::be transposed; int the freduencyssnec m .11 addition, resonant=fiim it 2'5, 28:,pt videsmeans connected to device 22 for deriving therefrom an output signal or intermodulation product corresponding to the input signal supplied through coil 20, but transposed in frequency. For the case unde consideration, where deyice 22 is employfid as the mixer or first detector of a superheterodyne receiver, the output signal is the usual intermodulation product produced by the heterodyning action of the mixer in response to the received signal delivered from antenna it, I] and the local signal supplied by oscillator 17.

One embodiment of the mixer found to have satistactory operating characteristics and emciency employed the following:

Element 22". (3-7 or CK710 germanium crystal Resistor :26 /2 m e hm Battery l8 200 volts The resistance of resistor 26 is very much greater than the impedance of diode 22, so that this resistor and voltage source it constitute a source of constant current in series with the diode element to establish a static bias current therein .in .the direction of greater conductivity in order to fix the operatin point of the diode at a particular part of its voltage-current ,characteristic.

The voltage-current characteristic for the diode is represented by a full line curve .3 0,oi Fig. 2 in which voltage is plotted as .the abscissa and current as theordinate. The bracketed .portion 3! of curve exhibits maximum curvature and inorder to attain highconversionefiiciency for modulator it it is desirable that element 22 be op 'ated along this portion 31 of its characteristic curve. With ,the illustrative values of source [3 and resistor .26 stated above, bias current of 0.5 inilliampere, represented by ,a horizontal dash line 32, is established in .diode .22, and a drop of 0.25 ,volt, represented by the .vertical dash line is developed across the .diode.

The intersection A of the bias-.Qurrent curve 3.2 with the characteristic .30 denotes the operating point of the diode which, preferably, corresponds to the vpoint of maximum curva- ..ture. Commercially available germaniumdiodes {do not all have .the identical current-voltage characteristics andit nay be assumed that their ,tamily of. curves fall .withinthe limiting accept- ,able values represented by curves ,34 and ,35 ,in Fig. 2. The static bias-current curve 32 intersects this family of characteristics atpoints ,of :,very nearly tl ie,same.curvatur c so that the .described arrangement operates satisfactorily end-effectively without need for compensating adjustments when the diode ,is replaced or re- ,newed. This highlydesirable practical advantage is not realized inprior arrangement semploying aconstant voltage as distinguished from ,a constant current .bias. ,From an inspection of Fig. 2 it is immediately apparent that the line .35 of constant voltage intersects curves 34 and 35 .at v points of widely different w curvature, d monst a i the de i n z i c n t n lta i biasfor the. diode.

Another advantage of the frequency v changer embodyi ethe p s n nvention el t d toa justmentiof the local signal intensity supplied .th tet .In .thamodulato s own .in .Eie- .1 excitation may be easily and,c onveniently m eas .ured,by, connecting a. D. ,C. ,volt neter across. dimen .wit no t ane i roneratio o ta I;- ili we e hi a mix iutili i is cltae bias; crystal excitation is measured by breaking the crystal circuit and inserting a current measuring device suitably by-passed for radio frequency signals. This procedure is not satisfactory if the stage operates at very high frequencies since the added circuit connections affect operation. Furthermore, since the mixer usually is at the input of a high-gain high-frequency amplifier, regeneration is likely to result from the addition of such connections.

The teachings of the invention are also applicable to a signal detector circuit for demodulating an amplitude-modulated wave as shown in Fig. 3. Incoming intermediate-frequency signals are supplied through a transformer 36 which is double-tuned in conventional fashion. One terminal of the secondary winding of transformer 36 is grounded and the other terminal is connected to the cathode of a germanium crystal diode 31 like element 22 of Fig. 1. The anode of diode 31 is connected to one terminal of the primary winding of an audio-frequency transformer 38, the other terminal of which is connected to the positive terminal of a potential source through a resistor 39. The junction of the primary winding of transformer 38 and resistor 39 is bypassed to ground by a condenser 40 having a low impedance to audio-frequency signals. Another bypass condenser 4| is connected between the anode of diode 31 and ground and has an impedance which is low for intermediate-frequency signals. The secondary winding of transformer 38 serves to couple the detector l4 with an audio amplifier (not shown).

Resistor 39 is chosen so that a constant current of approximately 0.5 milliampere flows through diode 3'! to establish operation thereof over a portion of its voltage-current characteristic of greatest curvature. As a consequence, the detector operates with high efficiency and is particularly suitable for use in applications wherein low signal levels are encountered.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

We claim:

'1. A wave-signal frequency-changing system comprising: a two-element, predominantly unidirectionally conductive crystal device having a non-linear voltage-current characteristic; 2. source of constant current comprising a source of potential and. a series impedance having a value very much greater than the operating impedance exhibited by said crystal device, connected in series with said device for establishing a static bias current therein to fix the operating point of said device at a particular part of its aforesaid characteristic; means for applying to said device a wave signal to be changed in the frequency spectrum; and means connected to said device for deriving therefrom an output signal corresponding to said wave signal, but changed in frequency.

2. A wave-signal frequency-changing system comprising: a two-element, predominantly unidirectionally conductive crystal device having a non-linear voltage-current characteristic, a portion of which is of maximum curvature; a source of constant current comprising a source of potential and a series impedance having a value very much greater than the operating impedance exhibited by said crystal device, connected in series with said device for establishing a static bias current therein to fix the operating point of said device within said portion of its aforesaid characteristic; means for applying to said device a wave signal to be changed in the frequency spectrum; and means connected to said device for deriving therefrom an output signal corresponding to said wave signal, but changed in frequency.

3. A wave-signal frequency-changing system comprising: a two-element, predominantly unidirectionally conductive frequency-changing device of the crystalline type having a non-linear voltage-current characteristic; a source of constant current, comprising a source of potential and a series impedance having a value very much greater than the operating impedance exhibited by said crystal device, connected in series with said device for establishing a static bias current therein in the direction of greater conductivity to fix the operating point of said device at a particular part of its aforesaid characteristic; means for applying to said device a wave signal to be changed in the frequency spectrum; and means connected to said device for deriving therefrom an output signal corresponding to said wave signal, but transposed in frequency.

4. A wave-signal frequency-changing system for a wave-signal receiver including at least one electron-discharge device and a source of space current therefor, comprising: a two-element, predominantly unidirectionally conductive crystal device having a non-linear voltage-current characteristic; an impedance having a value very much greater than the operating impedance exhibited by said crystal device connected in series with said crystal device and said source to constitute a source of constant current for establishing a static bias current in said device to fix the operating point of said device at a particular part of its aforesaid characteristic; means for applying to said device a wave signal to be changed in the frequency spectrum; and means connected to said device for deriving therefrom an output signal corresponding to said wave signal, but changed in frequency.

5. In combination: a two-element, predominantly unidirectionally conductive device of the crystalline type having a non-linear voltage-current characteristic; and a source of constant current, comprising a source of potential and a series impedance having a value very much greater than the operating impedance exhibited by said crystal device, connected in series with said device for establishing a static bias current therein to fix the operating point of said device at a particular part of its aforesaid characteristic.

JOHN F. BELL. ROGER M. NORDBY.

References Cited in the file Of this patent UNITED STATES PATENTS Name Date Ballantine et a1. Aug. 23, 1932 Sharpless Mar. 2, 1948 Number 

